It has previously been proposed to reheat food products employing induction heating of elements within the food container. Such techniques known in the prior art are exemplified by U.S. Pat. No. 4,020,310 entitled "Container for Inductively Heating Food" issued to Souder, Jr. et al. on Apr. 26, 1977, and U.S. Pat. No. 4,110,587 entitled "Method and Apparatus for Heating Food" issued to Souder, Jr. et al. on Aug. 29, 1978, which is a divisional application of U.S. Pat. No. 4,020,310.
In the systems exemplified by the above noted U.S. Patents an oscillatory signal is induced in a fixed induction coil. This oscillatory signal generates magnetic fields which cause an induced current within an electrically conductive material integrated within the container for storage of the food. This induced electric current causes self-heating of this electrically conductive material, thereby permitting warming of the food.
In a practical environment of this invention a movable cabinet includes a plurality of such fixed induction coils at numerous shelf locations. In the preferred embodiment these induction coils are disposed at less than all of the possible shelf locations. The interior space of this movable cabinet is ordinarily kept refrigerated. Food which is not to be heated is placed within containers which do not have the electrically conductive material. These containers may be placed any where within the cabinet even at shelf locations having one of the fixed induction coils. The food will then be cooled by the refrigeration of the movable cabinet. Food to be heated is placed within the specially formed containers having the electrically conductive material, these specially formed containers being further placed at a shelf location having one of the fixed induction coils. The induction heating coil induces a current within the electrically conductive material causing electrical self-heating and warming of the food. The degree to which food is heated is based upon the type of container, including the location and character of the electrically conductive material, if any, and upon whether or not the container is placed at a location having an induction coil. This technique enables food to be heated and food to be cooled to be kept in the same movable cabinet and does not require selective actuation of switches or the like to enable the heating.
A problem with such a technique for heating food is the stimulation of the oscillatory signal within the fixed induction coils. It would be of particular advantage to be able to transfer a maximum controlled amount of power from an electrical power source to such an induction coil. A typical system would include a capacitor connected to the induction coil in order to provide an LC oscillation circuit. Because of losses such a circuit must be periodically re-energized to ensure continued oscillation.
In accordance with the prior art it is known to periodically couple an electrical power supply to the LC oscillation circuit in order to re-energize this circuit. Bipolar or field effect switching transistors can be employed for this purpose. These devices have limitations. In particular, it has been found that the use of switching transistors carries a large risk of overcurrent destruction of these devices, particularly during the initial application of power to the LC oscillation circuit.
It is therefore a need in the art to be able to provide an induction heating driver circuit not having the limitations of the prior art.